In turbojet engines, the sealing gap between the rotating blades and the stationary engine housing represents a limiting variable which is of considerable importance for the efficiency of the engine. In order to minimize the sealing gap in turbines, it is known to provide the turbine with a shroud band which is attached to the blade tips. In the case of turbines with individually attached blades or blades attached in groups, the shroud band comprises band portions attached to the blades and having adjacent surfaces in toothed engagement with one another.
A shroud band constructed in this way is not possible with a turbine wheel having integral rotor blades. Conventional shroud bands for turbine wheels with integral rotor blades have a number of separation gaps for every three to five blades to compensate for expansions occurring during intermittent operation and to avoid development of additional stresses in the already highly stressed blades. These separation gaps extend substantially parallel to the chord lines of the adjacent blades in order to disconnect the shroud band from the blade tips in a way to minimize stress at the connection of the shroud band to the blade tips. In addition the separation gaps are formed with a short middle portion extending substantially at right angles to two end portions, so that the gaps have a substantially Z-shaped outline. The middle gap portion has a small width which is intended to be taken up, in operation, by thermal expansion, so that the adjacent shroud band portions are frictionally engaged at the middle portions of the gaps. In this way, vibration of the blades and of the shroud band portions connected thereto are effectively damped by friction, in which case the vibrations are additionally de-tuned. The two end or lateral portions of the gap (so-called free gaps) joined to the middle portion of the gap have sufficiently wide gap widths so that contact does not occur there under any conditions.
The conventional shroud band construction has the disadvantage that the individual middle gap portions cannot be produced with the small gap width desired to achieve optimum operating conditions, since the cutting tools used must have adequate rigidity and consequent thickness. For manufacturing reasons it is only possible to produce a finite gap which cannot be made less than from about 0.2 to 0.3 mm. This results in poorer damping and de-tuning characteristics of the vibrations which are developed. Consequently, vibrations of large amplitude occur, resulting in considerable stressing of the materials of the blades and shroud band.